1. Field of the Invention
This invention relates to a process for sequestering carbon dioxide through the production of lightweight controlled low-strength materials.
2. Description of the Related Art
Man made carbon dioxide (CO2) has been viewed by some as being a pollutant. There are many sources of CO2 including power plants, cement manufacturing, lime manufacturing, iron production, vehicles, natural gas ensuing from wells, ammonia manufacturing, fermentation, hydrogen production (e.g., in oil refining), and producing hydrogen fuels from carbon-rich feedstocks, such as natural gas, coal, and biomass. Thus, there has been interest in processes for sequestering carbon dioxide.
Permanent sequestration of carbon dioxide can occur by terrestrial ecosystems. For example, in forests and trees, 74,806 metric tons of carbon dioxide can be sequestered per 1,000-acre plantation in seven years. Also, grasses and prairies, such as the United States Great Plains, sequester 740 kg/ha/year. Agricultural and biomass croplands, boreal wetlands and peatlands in United States agricultural lands sequester carbon dioxide at rates of 75-200 million metric tons per year.
Permanent sequestration of CO2 can also occur by advanced chemical and biological sequestration wherein CO2 is converted into either commercial products that are inert and long lived or that are stable solid compounds. Examples include: magnesium carbonate, CO2 ice-like material, advanced catalysts for CO2 and CO conversion, engineered photosynthesis systems, non-photosynthetic mechanisms for CO2 fixation (methanogenesis and acetogenesis), genetic manipulation of agriculture and forests to enhance CO2 sequestering potential, advanced decarbonization systems, and biomimetic systems.
Permanent sequestration of carbon dioxide also occurs by the natural carbonation of concrete. About 0.2 pounds of carbon dioxide is absorbed per 1 pound of cement in concrete over time. During a typical year of concrete construction in the United States, 274,000 metric tons of atmospheric CO2 are absorbed. Over a 100-year period, all of the concrete produced during a single typical year will absorb 2,906,000 metric tons.
Various methods are known for storing carbon dioxide in geological media such as enhanced oil recovery, storage in depleted oil and gas reservoirs, replacement of methane by carbon dioxide in deep coal beds, injection into deep saline aquifers, and storage in salt caverns. Also, ocean sequestration of carbon dioxide is known. Carbon dioxide may be stored underwater in large canyons or injected directly into the deep ocean, via pipeline or tanker. Furthermore, oceans naturally sequester carbon dioxide. It has also been proposed to capture carbon dioxide by microalgae, ocean fertilization, or non-biological capture from the air.
Of course, various methods for reducing carbon dioxide production are also known. For example, power plant efficiency can be increased by way of: (i) processes, such as flue gas separation, oxy-fuel combustion, and pre-combustion separation, and (ii) systems, such as solvents (chemical, physical, and hybrid systems), membranes, cryogenic separation, solid-bed adsorbents, and combined systems. Carbon dioxide production can also be reduced by decreased manufacturing of cement through use of alternatives to conventional cement like fly ash. Decreased use of vehicles can also reduce carbon dioxide production.
It has been reported that in 2000, carbon dioxide emissions reached 5.8 billion metric tons and that approximately 80% of annual emissions come from the burning of fossil fuels. Therefore, there have been efforts to limit man made carbon dioxide emissions from fossil fuel burning plants. U.S. Pat. No. 6,235,092 provides a discussion of a variety of processes that have been developed for removing a gaseous component (such as carbon dioxide) from a multicomponent gaseous stream (such as the exhaust gas stream of a coal burning electrical power generation plant). Selective adsorption by solid adsorbents and gas absorption are named as two example processes. This patent further mentions that gas absorption finds use in the separation of CO2 from multicomponent gaseous streams. It is reported that in some CO2 gas absorption processes, the following steps are employed: (1) absorption of CO2 from the gaseous stream by a host solvent such as monoethanolamine; (2) removal of CO2 from the host solvent by steam stripping; and (3) compression of the stripped CO2 for disposal such as by sequestration through deposition in the deep ocean or ground aquifers. Other patents describing CO2 sequestration methods include U.S. Pat. Nos. 6,648,949, 6,372,023 and 5,397,553.
Although these processes may be successful in sequestering carbon dioxide, they can be energy intensive. Thus, there is continued interest in the development of less energy intensive processes for sequestering carbon dioxide from the exhaust gas streams of industrial and power generation plants.
It is also known that cement-kiln dust, lime-kiln dust, and slag cement are voluminous cementitious by-products of industry, presenting air quality and environmental disposal issues. Cement-kiln dust is a by-product of the manufacture of portland cement, and nearly 4 million tons of cement kiln dust are disposed of every year in the United States. Maintaining cement-kiln dust landfills is expensive and difficult because of the fine, dusty nature of the material, and disposal costs add significantly to the cost of cement manufacture. Cement manufacturers are making efforts to minimize the generation of cement kiln dust and to develop alternative uses for this material.
Lime-kiln dust is a by-product of the manufacture of lime in a rotary kiln. Lime-kiln dust has limited use as a raw material in cement production and is commonly used in agriculture as a soil stabilizer or soil-modifying agent. As with cement-kiln dust, manufacturers are making efforts to reduce disposal costs and environmental impact by minimizing the generation of lime-kiln dust and developing alternative uses for the material.
Slag cement is a by-product of iron production. When mixed with portland cement, it is used in concrete and other construction applications. The use of slag cement in concrete and cement mixes has economic and environmental benefits, reducing slag disposal costs, greenhouse gas emissions, and total energy use.
Controlled low-strength materials are described in the publication “Controlled Low-Strength Materials”, reported by American Concrete Institute Committee 229, June 1999, as self-compacted, cementitious materials used primarily as a backfill in place of compacted fill. Conventional CLSM mixtures consist of water, portland cement, fly ash, and, optionally, additional fine or coarse aggregates. However, CLSM mixtures can be made of other cementitious materials mixed with water. This publication defines CLSM as a material that results in a compressive strength of 8.3 MPa (1200 psi) or less at the conventional 28 day testing period (typically without compaction), and notes that most current CLSM applications require unconfined compressive strengths of 2.1 MPa (300 psi) or less at the conventional 28 day testing period in order to allow future excavation. The use of cement-kiln dust, lime-kiln dust, and/or slag cement in CLSM provides a environmentally beneficial use for these cementitious materials.
It is also known that the density of cement-containing materials can be lowered by entraining gas in the materials. For example, U.S. Pat. No. 3,867,159 describes cellular concrete structures made by mixing water and cement followed by the introduction of a foam produced by a foam generator. U.S. Pat. No. 4,383,862 discloses a method of producing aerated concrete wherein a gas such as air, carbon dioxide, or a mixture thereof is introduced into a blender with a mortar mix. U.S. Pat. No. 5,654,352 describes air-entraining agents for use in cementitious mixtures. Some of these mixtures are reported as being controlled low-strength materials. U.S. Pat. Nos. 6,153,005, 5,013,157, 4,900,359, 4,415,366 and 4,407,676 describe related processes.
It is also known that the hardening of cement-containing materials can be accelerated by carbonation in which calcium hydroxide in the cement is transformed into calcium carbonate by absorbing carbon dioxide. Related processes are described in U.S. Pat. Nos. 6,387,174, 6,264,736, 5,965,201, 5,897,704, 5,690,729, 5,650,562, 5,518,540, 5,307,876, 5,051,217, 4,427,610, 4,362,679, 4,350,567, 4,117,060, 4,093,690 and 4,069,063, German patent application DE 4207235, Swiss patent application CH 644828, and Japanese patent applications JP 6263562 and JP 2018368.
There is a continuing need to safely and economically dispose of such industrial by-products as cement-kiln dust, lime-kiln dust, and slag cement; thus, there is a continuing need for products and processes that make environmentally beneficial use of these by-products. In addition, increasing global concerns regarding greenhouse gas emissions have created a continuing need for products and processes that permanently remove free carbon dioxide from the environment.